Rotor spinning is a spinning process that produces genuine yarn twist. This „genuine“ twist, which is retained in the yarn, is decisive for yarn strength. However, in order to maintain the spinning process, i.e. a stable and reliable binding-in process, a spinning twist is required, as explained in the previous section, which must be higher than the yarn twist required for yarn strength. This means that additional twist must be created in the radial length of thread extending from the draw-off nozzle into the rotor groove. This additional twist, the so-called false twist, is created by the rolling motion of the yarn on the draw-off nozzle. Depending on spinning conditions, the false twist can amount to as much as 60% of the yarn twist set.
So how does this false twist effect arise and how does it differ from genuine yarn twist?
Genuine twist that is retained in the yarn (Fig. 96) is generated when a length of yarn is clamped at one end and rotated around its axis by a twisting element at the other end. Transferred to the spinning box of a rotor spinning machine, this means that the yarn is clamped by the take-off rollers and twist is imparted by the rotating rotor. One revolution of the rotor corresponds to one turn of the yarn. The genuine twist therefore corresponds to the required twist set. The number of required turns imparted to a yarn depends on how long the length of thread remains in the rotor; the longer this time, the higher the number of turns. This means that the ratio of delivery speed (in m/min) to rotor speed (rpm) defines the number of required turns set:
A nip and a twisting element are also required to generate false twist (Fig. 97), but an additional passive or active twist element is also required. If additional turns, i.e. false twist, are imparted to the yarn by this twist element, these are distributed to the left and right of the twist element in mutually opposing directions of twist (see Fig. 96). When the yarn leaves the nip the length of yarn twists back into its original form – by exactly the number of additionally inserted turns. This is precisely what happens in our rotor. The take-off rollers form the nip and the centrifugal force in the rotor groove acts as the twist-generating element; these two forces act in opposition to one another. The passive twist element in this case is the draw-off nozzle. The yarn is pressed onto the nozzle surface during take-off by the contrary tensile forces and unwinds on this surface. A certain number of additional turns – the false twist – are imparted to the yarn while it unwinds on the nozzle surface. The false twist effect created between the draw-off nozzle and the yarn unwinding on it has Z twist between the draw-off nozzle and the rotor groove, and S twist between the draw-off nozzle and the nip of the take-off rollers. The higher the friction on the nozzle surface, the higher the number of additional, reversible yarn turns inserted.
False twist, i.e. spinning tension, can be increased by:
- a larger nozzle surface diameter;
- additional notches, grooves, ridges, etc., arranged radially, axially or helically on the surface of the draw-off nozzle;
- a tighter bend in the thread draw-off tube; and
- additional twist accumulating elements in the bend of the thread draw-off tube.
During take-off, the yarn moves clockwise along the surface of the nozzle. In so doing, the yarn is twisted in the counterclockwise direction. The partial rolling of the yarn gives rise to false twist between the twisting-in point for the fibers and the nozzle. The yarn in the spinning section (b in Fig. 95) therefore exhibits more turns of twist than the spun yarn. Moreover, the twist level increases continuously from the nozzle toward the rotor groove. The twist level at the lift-off point is about 20-60 % higher than at the nozzle. This difference arises from variations in tension along the yarn. Yarn tension is generated by the take-off rollers during takeoff in opposition to the centrifugal force in the rotor. Tension is highest at the take-off rollers themselves and declines toward the rotor wall. However, yarn tension and twist level are inversely proportional, i.e. if there are sections of low tension in the yarn (c), these will exhibit more twist. On the other hand, sections of high tension (b) take up less twist.
It is only these additional turns at the lift-off point, caused by false twist and yarn-tension variations, that enable spinning to be performed under stable conditions. The falsetwist effect is dependent upon carrying along the yarn at the nozzle, i.e. ultimately upon the roughness and the structure of the contact surface. However, it also increases with increasing rotation speed of the rotor.
The angle of inclination of the fibers being twisted-in is the decisive factor for yarn tenacity. In order to achieve the same angle of inclination and thus the same level of tenacity, twice as many turns have to be imparted to a fine count yarn as to a yarn twice as thick. The absolute number of yarn turns only gives an indication of yarn tenacity if this is related to yarn count. However, twist multiplyer α/m or α/e enables the twist level of a yarn to be described regardless of yarn count. The higher the twist multiplyer, the higher the twist level and the higher the yarn tenacity, and vice versa. Yarn turns can thus be calculated as follows:
* Conversion factor dtex/Micronaire
Based on the fact that turns in rotor yarns are more inclined to move to the yarn core, while the yarn surface features a rather indifferent fiber layer and wrapper fibers, yarn twist can only be defined approximately in terms of measuring technology. In contrast to ring-spun yarn, rotor-spun yarn cannot be twisted until the fibers are completely parallel. That is to say, the number of turns measured is always lower than the required number of turns produced on the machine. The variances can be as much as -20% and depend mainly on the characteristics of the fiber staple – rectangular or triangular staple – and the number of wrapper fibers.